RETRACTABLE SYRINGE WITH A CLOSED BARREL

Abstract: A retractable syringe that pulls the needle and holder (202) from a fluid sealed releasable connection in the open forward end of the syringe barrel to a position enclosed within the barrel. A clip (204) may lock the holder to the barrel to prevent release. The piston shaft may then be broken off at the piston (38) and the free end of the shaft inserted through the now open forward end of the syringe barrel, enclosing and sealing the needle holder and the remaining fluid in the barrel, with the shaft holding the needle from longitudinal movement within the barrel. The syringe can then be disposed of without allowing access to the needle or having fluid leakage.

Description: RE RACTABLE SYRINGE WITH A CLOSED BARREL CROSS-REFERENCES TO RELATED APPLICATIONS This application is a Continuation-In-Part of application Serial No. 07/939,449, filed September 2, 1992, which is a Continuation-In-Part of application Serial No. 07/714,431, filed June 13, 1991, now U.S. Patent No. 5,205,824. BACKGROUND OF THE INVENTION The present day danger to doctors, medics and nurses resulting from using syringes is well known. But to a subsequent user, contagious diseases such as AIDS and hepatitis have made the reuse of a syringe a catastrophic event. Yet notwithstanding all of these dangers and problems, the common syringe that has been sold for years and is still used in volume, remains the same. While this syringe may use a removable sheath, this does not effectively protect against inadvertent needle pricks, that often occur in just trying to return a used hypodermic needle to the sheath. Also sheaths do not protect against the dangerous and highly injurious reuse of hypodermic needles. There are syringe designs that have the capability of retracting the needle or the needle holder into the syringe barrel, as exemplified in U.S. Patent Nos. 4,838,870 and 4,650,468 that retract the needle into the barrel, and in U.S. Patent Nos. 4,747,830 and 4,790,822 that retract the needle holder into the syringe barrel. The latter U.S. Patent Nos. 4,747,830 and 4,790,822 additionally disclose breaking off the end of the plunger shaft and holding the needle and holder in the barrel. The approach of retracting the needle or the needle housing into the syringe barrel has theoretical advantages. But in actual use the designs disclosed in the aforesaid patents have proven to be impractical in operation, are not cost effective as they are expensive to manufacture, and present problems in leaving substantial amounts of fluid in the barrel after the needle or needle holder have been retracted. Further the differences in the operation of the new designs relative to the old common syringe are so great that the safety factors achieved are not sufficient to justify their adoption. The patents disclosing withdrawing the needle into the barrel have very complex mechanisms that have the disadvantage of being costly to make and are basically impractical in operation. The mechanisms within the barrel volume for grasping and retracting the needle are so bulky it is difficult to sufficiently eject the fluid from the syringe barrel. So in retracting, substantial amounts of liquid are left in the barrel to leak out later. Even if this liquid is retained in the barrel, this still leaves the syringe in a less desirable condition for disposal. Essentially a syringe is a very simple device and it has to be kept simple ih construction and in operation as well as being inexpensive to make; or it will not achieve wide acceptance because of the large number of syringes that are used each year. The patents that disclose retracting the entire needle holder and needle into the barrel, likewise have very complex mechanisms for accomplishing the retraction that makes such syringes expensive to make and subject to operation failure. Some of these systems require that the forward end of the syringe be frangible. These syringes all are either not capable of being manufactured at a reasonable cost or cannot be broken easily. Further the frangible section makes the syringe structurally weak at a very critical point, and also makes the syringe unsealable after being broken. Some of these systems also require that the plunger shaft be broken to render the syringe inoperable and to assure that the syringe needle and holder cannot be pushed back out of the forward end of the barrel. The problem with this is that the plunger shafts are difficult to break in practical use. Even if the shaft can be broken, the effort in doing so is distasteful to the syringe user, because the syringe user has to be mindful of the fact that the unattached needle and needle holder can be caused to eject from the syringe barrel and cause an accidental pricking by the hypodermic needle. Further in the systems that retract the needle and holder into the barrel, after such retraction the barrel is essentially not closed at one end. So the fluid is released, making an obvious mess. The two systems design that retract the needle and holder into the barrel have a design that makes the syringe expensive and complex to make, and so the price and complexity makes the syringe design unlikely to be used in the future. There is therefore, a real and demanding need for a new and improved syringe that easily, quickly and positively retracts the needle and needle holder into a sealed condition within the syringe barrel, which has a means for easily breaking off the shaft from the piston rendering the syringe incapable of future use, and for providing a simple and easy means of sealing the open forward end of the syringe in locked manner so that the syringe needle and holder cannot inadvertently escape or be jarred out of the open end of the syringe barrel, and that provides a new and improved syringe that is relatively inexpensive to make, is simple and positive in operation, that is safe and easy in use, that allows maximum ejection of the fluid from the barrel in use, that locks the needle holder from being inadvertently withdrawn into the barrel, that retains the retracted syringe in the barrel and seals the syringe and the fluids in the barrel, and that can be disposed of in the sealed condition. SUMMARY OF THE INVENTION The present invention provides a retractable syringe in which the needle holder is releasably positioned and secured in the forward open end of the syringe barrel. The piston and the shaft operate as a plunger in the syringe, to draw in and dispense liquids in the normal manner. When the syringe has been used and the liquids dispensed, the plunger is forced in with slightly greater force than in normal operation to cause the piston to engage the needle holder. The plunger is then pulled rearwardly pulling the piston and the joined needle holder, releasing the releasable connection to the forward open end of the barrel. The combination piston and needle holder are then retracted to a position adjacent the rear opening of the syringe barrel, where the shaft is broken off at a weakened section adjacent to the piston in a quick and easy manner. The piston retains its position and seals the rear opening of the barrel and restrains the needle holder from movement out the rear end of the barrel. The free end of the shaft is then inserted into the open forward end of the barrel and sealing means attached to the shaft are positioned into the forward open end of the barrel, sealing the barrel and locking the shaft in a position that holds the needle holder from moving around in the barrel. All of this is accomplished in a simple and straightforward manner, using straightforward and understandable mechanisms, and that does not require major breakage of what are essentially difficult to break components within the syringe structure. The syringe thus ends up with the piston blocking and sealing one end of the barrel, and the shaft sealing and locking the other end of the barrel, with the shaft projecting into contact the needle or needle holder in the barrel, holding the needle from movement. The entire package may then be disposed of in a safe and simple manner. This does not require the use or reliance upon a sheath to cover the end of the hypodermic needle after it has been used, and does not allow the syringe and needle to be reused, and further restricts any contact with the needle or the fluid that was formerly within the syringe barrel, from being dispensed or spread around in the area of disposal of the used syringe. It is therefore an object of this invention to provide a new and improved retractable syringe that corrects the shortcomings of other prior retractable syringes. Other objects and many attended advantages of this invention will become more apparent upon a reading of the following detailed description and an examination of the drawings in which like figures designate like parts throughout and in which: BRIEF DESCRIPTION OF THE DRAWINGS •Figure 1 is a side elevation view of the retractable syringe, with portions cut away; Figure 2 is an enlarged sectional view taken on line 2-2 of Figure 1; Figure 3 is an enlarged sectional view taken on line 3-3 of Figure 1; Figure 4 is an enlarged sectional view taken on line 4-4 of Figure 1; Figure 5 is an enlarged sectional view taken on line 5-5 of Figure 1; Figure 6 is a perspective view of the piston end of the plunger; Figure 7 is a perspective view of the needle carrier; Figure 8 is a longitudinal sectional view of the syringe body, illustrating the initial insertion of the needle carrier; Figure 9 is a view similar to Figure 5, with the piston latched on the needle carrier; Figure 10 is a similar view, with the piston and shaft withdrawn and the needle carrier and needle retracted into the syringe body; Figure 11 is a similar view, with the piston shaft broken off and the piston and needle carrier locked in the retracted position; Figure 12 is a similar view, with the shaft inserted and locked in the opposite end of the body to enclose the needle; Figure 13 is a sectional view taken on line 13-13 of Figure 12; Figure 14 is a view similar to Figure 5, with a modified alignment means between the piston and the needle holder, and illustrating ejection of fluid; Figure 15 is a view similar to Figure 14, with the piston and needle holder interlocked; Figure 16 is a view similar to Figure 14, with an alternative piston ring and illustrating the drawing of fluid into the syringe; Figure 17 is an enlargement of a portion of Figure 16, with a detail of the piston ring; Figure 18 is a side elevation view of the structure of Figure 16, illustrating the use of the piston ring as a capacity indicator; Figure 19 is a side elevation view of a modified form of the piston; Figure 20 is a view similar to Figure 11, showing the piston retained in the syringe with the needle holder attached; Figure 21 is an end view of the syringe, illustrating the orientation of the plunger with the syringe body; Figure 22 is a side elevation view, partially sectioned, illustrating a pre-use sheath secured over the needle; Figure 23 is a similar view illustrating an alternative needle sheath arrangement; Figure 24 is a sectional view taken on line 24-24 of Figure 23; Figure 25 is a sectional view similar to Figure 17, with a different "O" ring; Figure 26 is a perspective view similar to Figure 6 with a different frangible section; Figure 27 is a side elevation view similar to Figure 19 with a different frangible section; Figure 28 is a sectional view taken on line 28-28 of Figure 27; Figure 29 is a sectional view taken on line 29-29 of Figure 16; Figure 30 is a side elevation view similar to Figure 1 with a modified barrel and shaft stabilization construction; Figure 31 is a similar view to Figures 11 and 31, with the piston and needle holder interconnecting structure being modified in a new embodiment; Figure 32 is a side elevation sectional view similar to that of Figure 5, illustrating the modified embodiment in Figure 31; Figure 33 is a view similar to Figure 5, showing an alternative structure for interlocking the piston and needle holder; Figure 34 is a sectional view taken on line 34-34 of Figure 33; Figure 35 is a view similar to Figure 33, showing the piston and needle holder interlocked; Figure 36 is similar to Figure 11, with the alternative structure of Figure 33; Figure 37 is a front end view of the structure of Figure 35, showing the action of the needle holder retaining C-clip; Figure 38 is an enlargement of a portion of Figure 33 illustrating the C-clip hinge pin connection with the barrel in more detail; Figure 39 is a view similar to the forward end of Figure 33, illustrating a modified C-clip connection to the barrel; Figure 40 is a front end view of the structure of Figure 39, showing the action of the C-clip; Figure 41 is an enlargement of a portion of Figure 39 illustrating the hinge connection in more detail; Figure 42 is a view similar to Figures 5 and 33, showing another alternative structure for interlocking the piston and needle holder; Figure 43 is an enlargement of the piston and needle holder interlock portion of Figure 42; Figure 44 is a view similar to Figure 43, showing the piston and needle holder interlocked; Figure 45 is an enlarged sectional view taken on line 45-45 of Figure 42; Figure 46 is an enlarged sectional view taken on line 46-46 of Figure 42; Figure 47 is an enlarged sectional view taken on line 47-47 of Figure 42; Figure 48 is an enlarged sectional view taken on line 48-48 of Figure 42; Figure 49 is an end view of the barrel of a syringe of the type illustrated in Figure 42; Figure 50 is an enlarged sectional view taken on line 50-50 of Figure 49; Figure 51 is a sectional view illustrating a retractable syringe of the type illustrated in Fig. 42 with the needle carrier locked in the retracted position and the shaft inserted and locked in the forward end of the barrel to enclose the needle; Figure 52 is a sectional view taken on line 52-52 of Figure 51; Figure 53 illustrates a portion of the forward end of a syringe having an alternative needle carrier structure; Figure 54 is an end view of the syringe illustrated in Figure 53; Figure 55 illustrates the forward portion of a syringe of the type illustrated in Figures 53-54, showing a shaft inserted and locked in the forward end of the barrel; Figure 56 is a sectional view of the forward end of a 5 syringe of the type illustrated in Figures 53-55, showing a needle integrally formed with the needle carrier; Figure 57 is an enlarged view of the piston and needle holder interlock portion of Figures 43; Figure 58 is an enlarged view of the piston and needle 10 holder interlock portion of Figure 44; and Figure 59 is an enlarged, partially sectional view of the forward end of the syringe shown in Figure 42. DESCRIPTION OF THE PREFERRED EMBODIMENTS 15 Referring now to Figure 1, there is illustrated a retractable syringe 10 having a hollow syringe barrel 12. This barrel and other suitable parts of the syringe are made of a plastic material such as polypropylene. The 20. syringe barrel 12 has a forward end 15 and a rearward open end 17. Releasably positioned in the forward end is the needle holder assembly 18. A movable plunger has a piston 24 with an O-ring seal that is connected to a shaft 14 that extends out the open end 17 of the barrel 12. A thumb 25 driven actuator surface 16 at the end of the shaft 14 coacts with finger grip 26 on the open end of the barrel 17 to push and pull the piston 24 in the barrel in the manner of operating the syringe. The piston 24 has means for grasping the end 32 of the needle holder 18, which will be 30 described in more detail hereinafter, and pulls the needle holder rearwardly to the point that the needle holder 18, the needle housing 20 and the needle 22 are all moved into a retracted position within the syringe barrel as illustrated in Figure 11. With the needle holder in this 35 position, the needle 22 is entirely enclosed within the volume of the barrel 12. The shaft 14 is then broken off from the piston 24, and the free broken end of the shaft 14 is then inserted through the forward open end 15 of the barrel 12. The shaft thus functions to close off the open forward end of the barrel with the piston closing off the rearward open end of the barrel, securing and enclosing the retracted needle within the barrel. Both ends of the barrel are thus closed and are sealed by the piston at one end and the shaft at the other end, so that the needle and the remaining fluid are both sealed within the barrel. The barrel is now ready for disposal. In other embodiments, the piston and joined needle holder can be locked in the barrel, or are prevented from being pulled out the rear open end of the barrel. A separate "C" clip lock means prevents the needle holder from being moved from the releasable connection until released by the syringe user. Also the space between the barrel and the needle holder is sealed by an "O" ring seal. In still another embodiment, see Figures 31 and 32, the rear end of the needle holder 152 has the grasping means, fingers 148 and 150. The adjacent forward end of the piston 142 has a projection with a circumferential slot 144. The fingers grip the end of the piston 142, and locks the piston and the needle holder together. Then pulling the plunger rearwardly pulls the piston 142 and the secured needle holder 152, from the open end of the syringe to the retracted position as previously described. Referring now to Figures 1, 5, 6 and 9, the piston 24 has an outer circumferential groove 49 for receiving an 0- ring 48. O-ring 48 may be made of any suitable material and functions as a movable seal between the piston and the inner surface of the barrel 12. The • • O" ring is preferably made of rubber. It has been discovered that rubber functions as a better seal against the plastic barrel, than plastic against plastic. The groove 49 has sufficient width and depth to prevent the O-ring 48 from rolling over or out of groove 49. Thus the O-ring 48 functions as a wiper for wiping inner surfaces of the barrel along with moving the fluid by virtue of the piston action both into and out of the hollow syringe barrel 12. The piston 24 has an axially centered recess 56 that receives and co-acts, in one embodiment of the syringe, with a nut type alignment 94, see Figures 14, 15 and 16, as will be described in more detail hereinafter. The front end of the piston 24 has a pair of arcuate members 30 and 31 each of which form fingers with internal shoulders 33 that co-act to slide over the rear cam surface 27 on the rear end of the needle holder 18. The fingers 30 and 31, see Figure 3, have sufficient resilient biasing to cause the fingers to be cammed outwardly by the cam surface 27 to snap in, fit in or grip into the groove 32 of the needle holder 18. However, the fingers 30 and 31 are only arcuate sections, and do not enclose the entire circumference of groove 32, for reasons that will be explained in more detail hereinafter. The piston also has a circumferential slot 65 between a pair of rims 57 and 59, that have an outer diameter slightly smaller than the inner diameter of the barrel. These rims 57 and 59 provide longitudinal stability to movement of the piston and also for other functions that will be described hereinafter. The piston 24 is connected to a piston shaft 14 by a pair of radially spaced frangible members 37 and 38, see Figures 4 and 5, that in turn are connected to a plurality of interconnected lath type members that are formed into a substantially outer rectangular configuration that corresponds with the inner surface of the barrel. The composite interconnected lath type members forms a circumferential cross sectional area, see Figure 2, with each of the lath members being connected along their longitudinal adjacent edges. Lath member 90 is connected to the side edge of lath member 86 that is in turn connected to lath member 88 that is connected to lath member 87 that has a side projecting lath member 89. It may be understood that this entire composition shaft structure 40 provides lateral rigidity against bending force moving the piston 24. This configuration provides an open center space in the syringe barrel 12 for receiving the end of the needle and needle housing 20 when the shaft 24 is inserted into the open end of the barrel 15 as illustrated in Figure 12. Collars or discs 34 and 42 are molded into the shaft 14 and extend outwardly, see Figures 1 and 2, to provide centralized positioning of the shaft in the barrel 12. The frangible sections comprise lath type members 37 and 38, see Figures 1, 4, 5 and 6, that are radially aligned and have a relatively thin cross-section. These lath members may be broken when subjected to lateral bending forces in a given direction. These lateral bending forces are resisted by the other cross sectional configuration of the lath member 14 as previously described and as illustrated in Figure 2. The lath members 37 and 38 are also scored at 39 on either side or both sides of each member, which further facilitates the breaking of the frangible sections 37 and 38. The frangible members interconnect the remainder of the shaft with the rear side of the piston 24, thus allowing shaft 24 to be broken at a point adjacent to the piston. In another embodiment, see Figures 26, 27 and 28, a modified frangible section is substituted for the lath members 37 and 38, primarily to provide added lateral stability. In this embodiment the lath members 138 and 140 corresponding respectively to lath members 37 and 38, and are made with an M L" shaped cross-section. The bottom of the M L" shape for each section is integral with the adjoining lath members 86 and 87. Each of the H L" shaped members are scored at 139 and 141 on all sides except the outer edges 151, to facilitate breakage of the shaft in the manner previously described. The rear open end 17 of the barrel 12 has a V-shaped inner surface 37 that narrows the diameter of the opening. The respective collars 34 and 42 have a smaller diameter than the inner diameter of ring surface 36, and thus pass freely therethrough. However, the piston 24 has larger diameter rims 57 and 59 that restrain the piston from movement out of the open end 17 and through the inward V- shaped ring surface 36. In one embodiment, see Figures 9 and 10, the diameter of rim 57 is slightly larger than the inner diameter of ring surface 36 and rim 57 has an angled, canted outer edge surface that allows rim 57 to contact and be cammed through the V-shaped ring 36. Thus the inward V- shaped projection 36 snaps into the slot 65. The outer diameter and shape of piston rim 59 is relatively square and thus the outer side edges of rim 59 contact the side surfaces of the V-shaped projection 36 and prevents passage through opening 17. This effectively locks the piston into the open end 17 of the rear portion of the barrel 12. Thus a pulling force on end plate 16 pulls the plunger, and thus the shaft 14 through the open end 17, and also pulls the piston. The end rim 57 will contact the side of the V-shape projection 36. With a slight increase in force, rim member 57 can be pulled through opening 17 camming the canted surface of member 57 through the opening of the inner surface of the V-shaped ring 36. This causes the V-shaped ring to be secured in the manner previously described, and thus locks and holds piston 24 in the opening 36. The resilient pressure of the outer edges of the slot 65 against the sides of the inner ring surface 36 is sufficient to provide a fluid seal against fluid movement out of the open end 17. Further, O-ring 48 also functions to prevent outward fluid movement from the barrel 12 through open end 17. It may be understood that the piston 24 may be initially inserted into the barrel 12 by pushing the piston through the restricted neck 36 in the open end 17 of the barrel. This is accomplished by exerting considerable longitudinal force on the piston by the shaft 24. This force is sufficient to move the piston and the outer rim portions 57 and 59 through the neck 36. However, when the neck 36 is snapped into the slot 65, this essentially locks the piston in the secured and sealed position where it is held against the normal application of forces against the piston that would occur in normal operation of the syringe. While the force required to move the piston from this locked position, as for example that force required to insert the piston through the neck 39 in initial installation, would normally not occur in the operation of the syringe. In another embodiment, see Figures 19 and 20, the piston has a different shaped rim member 92 that does not have the canted outer end surface and also has an outside diameter generally corresponding to the diameter of rim 59. So in rearward movement of the piston, the outer edge surface of rim 92 contacts the side of the inner shape ring 36, see Figure 20, in the manner that prevents movement of the ring 92 of the piston out of the open end 17 and through the inner ring-shaped projection 36. Accordingly, ring projection 36 does not snap into slot 65, and the piston is not locked into the open rear end of the barrel in .this embodiment. Rather, the piston is pulled to the end of the open end of the barrel, and it remains in that position. It may be understood that when the shaft 14 is broken off, O-ring 88 exerts sufficient outward pressure against the inner surface of the syringe barrel 12 to hold the piston 24 against longitudinal movement within the barrel, and to provide a seal against fluid movement. The needle 22 is held by a needle housing 20 in the known manner. The needle holder 18 has an internal threaded recess in its forward end for receiving the standard end 64 of a needle housing 20. The needle housing 20 is threaded into the threads 60 and is held in position. The wide space between the thread bights in threads 60 allows the needle holder 18 to hold or interconnected different, standard needle housing designs. Needle holder 18 has a conical forward outer end surface with a first circumferential groove 68. Groove 68 has a forward canted cam surface 82 and a rear abutment section 28 that forms one side of an O-ring recess for holding O-ring 50 in position. The forward open end 15 of the barrel has a reduced diameter end 51 that terminates into a neck with a lip 58 that has an internal circular hook portion 78 that fits into the groove 82 in the needle holder. The outer internal edge of the lip 58 has an outwardly canted rim surface that coacts with the cam surface 82 to provide a releasable latch for holding needle holder 18 in position. It may be understood that in inserting the holder 18 into the open end 15 of the barrel 12, the canted surface on the forward end of the holder 18 contacts the inner surface of the lip 58 and cams through the opening to the latched position. The material from which the barrel is made is slightly flexible with a resilient memory that returns the flexed lips 58 to the original position. This causes lips 58 to snap into the slot 82, locking the surface of the ring slot 28 and retaining the needle holder against longitudinal movement in the open end 15 of the. barrel 12. When the needle holder 18 is pulled rearwardly into the barrel 12 as will be described hereinafter, the cammjed surfaces of lip 58 are cammed by the canted surface of recess 82, allowing the needle holder 18 to be pulled through the opening 15 and into the barrel 12. The O-ring 50 provides a fluid seal in opening end 15. Again, preferably the O-ring is made of rubber. Also, the O-ring seal maintains a fluid seal around the needle holder end during slight rearward movements of the needle holder. The needle holder 18 has an axially positioned fluid passage 72 that connects to the fluid passage of the needle housing 20 and needle 22. Channel 72 is connected to a radial channel or passage 52 that conducts fluid into and out of the volume of barrel 12. When the piston 24 is pulled in the direction of arrow 118 a vacuum is drawn by the piston in the chamber volume 46. This draws fluid through the needle 22, passage 72, passage 52 into the volume 46 and fills the syringe barrel with fluid. In the fluid ejection or injection mode, the end 16 of the shaft 14 is pushed toward the finger hold 26 causing the shaft 14 to move the piston head 24 in the direction of arrow 96, see Figure 4, which causes the fluid to flow out through radial passage 52 and through passage 72 and out the end of needle 22. At the end of the discharge stroke, the longitudinal pressure on the shaft may be increased and continued causing the fingers 31 to cam over the end surface 32 of the rear end of the needle holder 18, and thus hook or grasp into the circumferential slot 32. The fingers 30 in grasping the needle holder 18, secures the needle holder to the piston 24. So when the piston 24 is pulled rearwardly, the piston pulls the needle holder with it, causing the needle holder to cam open the lips 58 of the open end 15 of the barrel. This pulls the needle holder 20 and needle 22 into the enclosed position as illustrated in Figure 11. The increased force required to cause the fingers of the piston to grasp the needle holder is of such magnitude that the syringe user has to intentionally and thus knowingly increase the force. - So accidentally joining the piston to the needle holder doesn't normally occur. In operation of the syringe it is desirable to be able to view the fluid in the barrel, and particularly the forward end of the barrel volume 46. The barrel in this invention is transparent, so this volume may be observed by looking through the side of the barrel and particularly at the passageway 52, to determine whether there is air or fluid in the forward end of the barrel, or whether the air has been bled out of the barrel. So in removing air bubbles from the barrel 10, the needle is directed upwardly and the piston 24 is moved to cause fluid to pass through the needle 22 thus ejecting the air. In this operation it is necessary that the volume around passage 52 be observable to assure that no air has collected in the end of the barrel. So it's also necessary that the grasping fingers 30 and 31 are aligned as in Figure 1, so as not to block viewing the volume 48 and passageway 52. Accordingly in initial construction of the syringe, the plunger i.e. the piston is aligned through rotating the thumb pressing member 16, which aligns the fingers 30 and 31 in the groove so that they do not shield the opening of passage 52. Only a single passageway 52 is shown to conduct fluid from volume 48 to passageway 72. It may be understood that there may be more than one of such passageways through the end of the needle holder, but the preferred embodiment has only a single passageway as it has been found that a single passageway is better capable of conducting fluid and air from volume 48 out through passageway 72, and not trapping or retaining an air bubble in the upper end of the volume 48, in operation of the syringe. In other embodiments of the invention, see Figures 31 and 32, the barrel 12 has a straight inner cylindrical surface without the inward V projection 36 of Figure 1. In this embodiment, the barrel is dimpled 47 by a dimple tool to provide an inward projection 45. It may be understood that the material from which the barrel 12 is made is of a very tough, durable, and yet slightly ductile, resilient material. Accordingly a known dimple machine may strike the side of the barrel 12, impacting the side of the barrel and causing a dent or dimple 47 that results in an inward projection or dimple projection 45. Thus, see Figure 31, the outer configuration of the rear ring member 157 of the piston 24, which has the configuration as illustrated in Figure 20 as ring 57, moves into an abutting contact with the inward projection 45, preventing the piston from being withdrawn from the barrel 12 without exerting a very large and intentional forces that would be obvious to the user. This dimpling functions to prevent the piston from being pulled out of the barrel 12 in normal operation. Also in this embodiment, the discs 34 and 42 have a diameter slightly less than the distance between the dimples, and accordingly the two discs 34 and 42 pass through the opening between the inner projections 45. This also applies to the ring or disc member 45. Ring or disc member 45 is a second collar or disc member that it secured to the shaft at a location closer to the end of the shaft, and which ring 45 functions to provide added stability against lateral movement by the shaft in its longitudinal movement in the volume 46 of the barrel 42. In another feature of the invention, the needle holder 18 has a slot 67 in circumferential groove 68 that receives the key 54 that is molded into the lips 58 of the barrel 12, see Figures 1, 11 and 20. When assembled, the key 54 is fitted into key slot 67 by positioning means 92 or 94, that rotate the needle holder relative to the barrel. This locks the needle holder in position and prevents rotation of the needle holder 18 when screwing the end of the needle housing into the internal threaded recess 64. This also allows alignment of the passageway hole 52 with the barrel 12 so that the finger grip 26 can be aligned with the bars 62 on the thumb presser 16 to correctly orient the fingers 30 and 31 with the needle holder 18 and the passageway 52. In inserting the holder 18 into the open end of the forward end 15 of the barrel 12, a tool such as a male key driver fits into and coacts with the shape of recess 92 in the rear end of the needle holder 18. This aids in moving the forward end of the needle holder through the lips 58 and also allows needle holder 18 to be rotated to the point that key 54 fits into slot 67 of the needle holder 18. In another embodiment, a nut-shaped end 94, see Figures 14 and 15, projects outwardly from the rear end of needle holder 18. A suitable wrench then fits over nut 94 and is rotated thereby, which aids in pushing the needle holder 18 into position and also to rotate and orient slot 67 with the key 54. Further, end 54 substantially fills the volume 56 in the end of piston 24, reducing the amount of fluid left in volume 46 of the barrel 12 after the fluid has been ejected through the needle 22, and also after the piston has been latched to the needle holder to pull the needle holder and needle into the barrel 12. In pulling the needle holder and needle into the barrel 12, it is desirable to eject as much fluid as possible from the syringe volume. In still another embodiment, the O-ring 48 in piston 24 comprises a split O-ring, see Figures 16 and 17. The 0- ring slot 121, which preferably is made of O-ring rubber, is larger than the radial slot 49 in Figure 14 and the 0- ring 120 has a wider, rectangular cross-sectional shape. The outer circumferential surface of O-ring 126 has a groove 122. This groove provides a pair of outer contacting surfaces 124 and 126 that provide a separate spaced wiping surface combination for wiping and sealing fluid moved by piston 24. A slanted edge surface 128 on one end, aids in restricting roll-up of the rearward edge of the O-ring 120, when pulling a vacuum in volume 46. Further, the circumferential aperture 122 has a defined edge 123 that co-acts with calibrated metering lines 130 on the barrel 12 to allow measurement of fluid in the barrel 12. In a modified embodiment, see Figure 25, the O-ring 136 is substituted for O-ring 120 on piston 24. O-ring 136 provides the same operational advantages as O-ring 126 and is symmetrical, simplifying installation. It may be understood that the side edges of the O-ring 120 as well as O-ring 48 are such that it restricts rolling movement of the O-ring 48 with movement of the piston 24. The advantage of the split O-ring 120 in Figures 16 and 17 is that it further restricts such rolling movement, and also provides a double seal along with providing a relatively visible line on which to calibrate the fluid in the barrel relative to the position of piston 24. Also the wider contact surface of the split O-ring 126 provides a wider contact surface with the barrel. This increases the holding force on the piston when the piston is in the retracted position and after the shaft has been broken, and the piston is not locked into the open end 17 of the barrel. Further this wider and larger surface contact also holds the piston against side movement and thus creates a longer force holding the needle against dropping by gravity when the piston and needle are retracted in the barrel and the shaft has been broken. Referring to Figures 1 and 12, when the shaft has been broken, and the free end has been inserted into the open forward end 15 of the barrel 12, the two collars or disc members 34 and 42 are thus caused to lock the shaft into the neck or lips 58 of the open barrel end. The disc 34, has a solid construction, see Figure 2, and has a canted surface. This canted surface when contacting lips 58, allows the disc 34 to pass through the opening. This then places the lips 58 in a locked position in space 44, and provides a sealing surface contact with the sides of discs 34 and 42. Further, since both discs are solid a double seal occurs between disc 34 and disc 42. Also encompassed within the inventor's invention is a needle coyer or syringe cover 100. This cover has a closed end 102 with an open end 103 having an internal configuration for fitting over and against the conical outer surface of the needle holder 18, and has a radial end surface at ring shoulder 106 that abuts against the forward surface of the lip 58 of the barrel 12. If an impact occurs against the end of the needle cover or sheath 100, this force is transmitted directly against the forward end of the barrel 12 and does not exert a rearward force against the needle holder 18, that could otherwise cause the needle holder to be released by the lip 58. In another embodiment, a third groove 110 is provided in the end of the needle holder, see Figures 23 and 24. This groove 110 receives a snap, half ring or C-clip 108 that is shaped to fit into the groove 110 and be retained in the position by its circular configuration that is slightly larger in circumference than one-half of the circumference of the groove. The snap ring has an open section that allows the ring to be inserted laterally into groove 110. In this position, the needle housing 118 is restrained from any rearward movement that would cause the housing 118 to cam through the lip latching means 58 at the end 17 of the barrel 12. This allows a standard needle sheath 114 to be mounted onto the needle holder 120. The retention clip 112 has an outward projecting knob 116 that is usually pushed circumferentially by the thumb of a user, which snaps the snap ring 112 out of groove 110. This releases the needle housing to be pulled by the piston into the barrel of the syringe. In operation of the syringe with a C-clip, the syringe user normally first expels air from the syringe by pushing the piston forward in the barrel. In this operation, an error could occur where the syringe user pushed the barrel too far and with too much force, causing the piston and needle holder to become engaged. With the C-clip in place, the user can exert even greater force in pulling the piston rearward, and with the needle holder held by the locking C- clip, the fingers cam outwardly allowing the piston to disengage from the needle holder, returning the syringe to its original condition, where it is ready for use. Further in normal operation of the syringe, the user usually inserts the syringe and makes the injection. After the injection has been made, and the syringe user pushes the plunger in with the syringe user's right thumb, the syringe user normally grasps the side of the barrel with their left hand, steadying the syringe as it is pulled from the person who received the injected fluid. When the syringe and needle is withdrawn from making the injection, then the syringe user can just push the plunger forward with force sufficient to engage the plunger and the needle holder. With the user's left hand on the barrel, the user merely moves their thumb forward and in flipping the thumb against the knob, flips the C-clip out of the slot or groove. Thus the needle holder is now released from the C- clip lock, and may be pulled with rearward force on the piston, to the retracted position in the barrel. IN OPERATION In assembly, the barrel 12 and the syringe 10 receives the needle housing 18 through the opening in the rear end 17 of the barrel 12. Any suitable tool pushes the needle housing 18 through the lip 58, which causes the lip to be 5 resiliently biased into the outer groove of the housing 18, thus releasably holding the housing 18 in position. A suitable wrench which may also be the driver for inserting the housing 18 into the barrel and into position, is used to rotate the housing 18 to the correct orientation as set 10 by the key 54 and the slot 68. The shaft 14 and piston 24 are then rotated by the finger pressure end 16 to correct alignment with the finger actuating flanges 26, to align the hole passage 52 with fingers 30 and 31. The shaft 14 thus drives the piston 48 into the 15 cylinder 12 to a location immediately adjacent the rear end 27 of the housing 18. The syringe piston 24 is then moved to draw fluid through the needle 22 into the volume 46 of the barrel 12. After the fluid is drawn into the barrel volume 46, the plunger is then pushed forward by the finge # r • 20 thumb pressure end 16 and shaft 14, with fluid being exited through the needle 22, to remove air from the piston volume 46. The fluid then flows through passage 52 to the inner passage 72 and out the needle 22. Fluid is then injected in the normal operation of the syringe with the piston 24 25 moving the measured amount as determined by the calibration units on the barrel, see Figure 18. When the desired fluid has been ejected through the needle 22, then any remaining fluid is further ejected by movement of the piston 24 into contact with the end 32 of the needle housing 18, with the 30 fingers 30 and 31 engaging the end of the needle housing 18. In this latched integral condition, the plunger 24 is then pulled by pulling on the shaft 14, in the rearward direction. This force is sufficient to cause the lips 58 to be cammed out of the slot 68, thus releasing the needle 35 housing 18 to be pulled through the opening in end 15 to the internal retained position where the needle is totally enclosed within the barrel 12. In one embodiment, the piston is pulled into contact with the end of the restricted diameter of the rear end of the barrel 12, see Figure 11, or in the embodiment in Figure 10, greater force is exerted onto the shaft 14 and piston 24, pulling it into the locked position of Figure 10. In either position, the weakened portions 37 and 38 of the shaft member 14 are at the end opening of the syringe barrel 12. The shaft 14 is then bent laterally breaking the shaft connection to the piston 24, thus breaking members 37 and 38 which may be along the score lines 39. The shaft 14 is then free from the piston 24, and is inserted into the now open end 17 of the barrel 12, see Figure 12. The shaft is inserted directly into the barrel with the needle 22 being positioned in the center space of the composite shaft structure, see Figure 13. The slanted radial end of the collar 54 cams through the lip opening 58. The lip 58 is then secured in slot 44, as the outer diameter of collar 42 is greater than that of collar 34. This, locks the shaft 14 into the open end of the barrel 12. Since the collars 34 and 42 are solid, a fluid seal also is created by the contact between collars 34 and 42 with the lip 58. It may be understood, that in this position the needle 22 may by virtue of gravity have a tendency to not hold its central position. However, the central space 84 in the shaft 14 is open to one side, allowing the needle to fit into the space 84, regardless of its position. It may be understood that the O-ring 48, see Figure 11, also seals the other end of the barrel 12, thus holding the needle 22 in a fluid sealed barrel 12 and being restrained from movement longitudinally at both ends of the barrel 12. In this condition, the entire unit is then disposed of, rendering the syringe incapable of further use, and also prevents the needle from contact or inadvertently penetrating or pricking the skin of the person using the syringe. Thus the person using the syringe is protected against inadvertent happenings with the syringe, and also allows the syringe to be disposed of in a manner that prevents fluid loss, other similar messes, and also in a manner that renders the syringe incapable of reuse. As further protection, the needle is provided with a sheath that is positionable over the needle and the needle 5 holder and housing, and will absorb any type of impact force on the end of the syringe, without causing the needle holder to be released and moved into the internal volume of the syringe. In another embodiment, an additional lock clip is used to also prevent inadvertent forces being 10 applied to the syringe that would cause the syringe holder to be inadvertently moved longitudinally into the barrel of the syringe. In one embodiment, the holder is removed from the syringe, and the syringe is immediately used in the manner previously described. In the second embodiment, a 15 standard syringe sheath is removed from the end of the syringe, allowing the syringe to be used in the normal manner. Then before retraction, the lock clip is removed from the forward end of the syringe, and the syringe is then retracted in the manner previously described. 20. In another embodiment, see Figures 31 and 32, piston 24 and needle holder 18 as previously illustrated in Figures 1 and 5, are modified to new piston 142 and new needle holder 152. The new piston 142 does not have the grasping fingers, but instead has a projection having an 25 outer frusto-conical end 146 with an inner forward outer circumferential slot 144. This end projection cooperates with the respective fingers 148 and 150 on the rearward end of the needle holder 152, that correspond with the fingers 30 and 31 previously described relative to Figure 1. The 30 fingers have the same shape as illustrated in Figure 3, and are cammed outwardly by the frusto-conical surface 146 to spring the fingers 148 and 150 apart so that the ends of the fingers pass into the slot 144, securing the piston 142 to the needle holder 152. Force is then exerted onto the 35 piston 142 to pull the piston 142 rearwardly. This in turn pulls the needle holder 152 in the manner previously described, to the retracted position. The inner rearward surface 162 of the needle holder 152 provides a cavity for collection of the fluid in the syringe, which fluid passes out through opening 166 to the needle 22. The inward canted surface 164 on the inner surface of the barrel 12, functions to help position the needle holder 152 when inserting the needle holder 152 into the retained position, and also to direct the fluid towards the flat surface 162 and thus into a central passageway 166. When the C-clip 108 is in position, the needle holder cannot be moved rearwardly into the barrel, either inadvertently or intentionally. For example, in operation of the syringe the user may force the piston by pressing on end 116 with sufficient force that it causes the fingers 30 or 31 to pass into the rear slot 53 and then secure the piston and needle holder together. This could render the syringe inoperable, other than to pull the needle holder out of the end of the barrel and secure the entire syringe into the inoperable condition as previously described. However, with the C-clip in position, the operator merely exerts a larger pulling force on end member 16, pulling the plunger sufficiently to cause the fingers 30 and 31 to release their grip on the groove 52, separating the piston from the needle holder. This returns the syringe to its starting condition. The increased force required to engage the piston with the needle holder is sufficient that in normal operation the user would recognize that the piston was being forced into a locking position with the needle holder. Yet even if this were to occur, the piston and the needle holder can still be separated by exerting sufficient force to pull the fingers out of the groove 52. The fingers can be pulled out of groove 52 and the fingers are constructed of a plastic that is tough and yet has a degree of flexible resilience that allows the fingers to be cammed outwardly to release the needle holder and locking connection with the fingers 30 and 31 of the piston 24. Regardless of the force exerted on the plunger to pull the plunger from connection with the needle holder, the force will not be sufficient to overcome the locking abutment of the C-clip. This same operation occurs relative to the embodiments illustrated in Figures 31 and 32, where fingers 148 and 150 will be pulled out of the slot 160 of the projection 144. Figures 33-38 illustrate another modified embodiment of the invention. In this embodiment, some parts are equivalent to parts in the previously illustrated embodiments, and like reference numerals have been used where appropriate. As illustrated in Figures 33-38, the retractable syringe 200 of this embodiment basically comprises a hollow syringe barrel 12 having forward and rearward open ends 15 and 17. A needle holder 202 is releasably locked in the forward open end of the housing 12 by means of releasable locking C-clip 204 which is hinged to the front end of the housing via hinge pin 206, as will be described in more detail below with reference to Figure 37. Needle holder 202 has an internal threaded recess 60 at its forward end for receiving a standard end 64 of a needle housing 20 from which needle 22 projects. A plunger 208 includes a shaft 14 projecting into the opposite rearward end 17 of the housing with an enlarged piston or head 210 at its forward end which is a sliding fit in the barrel for reciprocal movement back and forth in the medication chamber 212 inside barrel 12. The plunger 208 has an enlarged flange or thumb-driven actuator surface 16 at its rear end which coacts with finger grip 26 at the open rearward end of the barrel to allow the operator to reciprocate the plunger in the barrel in the desired fashion, in order to draw medication into chamber 212 via needle 22 and eject medication back out through the needle. The plunger 208 will now be described in more detail with reference to Figures 33 and 37. As illustrated in Figure 36, the enlarged head or piston 210 is secured to the remainder of the shaft 14 via a pair of frangible members 37, 38 identical to those described in connection with Figures 4 and 6 in the preceding embodiments. The remainder of the shaft is identical to that of the previous embodiment and includes elongated, interconnected lath members 86, 87, 88 and 90 which are equivalent to the lath members described above in connection with Figures 2 and 6 5 and extend up to actuator surface 16. A pair of ring members or collars 34, 42 are located on the shaft between the piston head and surface 16. These are of slightly smaller dimensions than the piston head 210 and operate to center the shaft as the piston reciprocates in the chamber. 10 Piston 210 is different from the previous embodiments and includes a pair of spaced enlarged front and rear discs or collars 214,216 with a reduced diameter portion 218 extending between the collars and forming an annular groove 220 for receiving double O-ring seal 222 which is similar 15 to the O-ring seal of Figure 25 above. In this embodiment, piston 210 has no slot for snapping into a ring at the rear end of the barrel, but instead the rear open end 17 includes a reduced diameter portion 224 with a ramped or inclined shoulder 226 facing chamber 212 which acts as a ,20 stop against the rear collar 216 to prevent the piston from being withdrawn from the barrel, as illustrated in Figure 36. The double O-ring seal 222 having an annular V-shaped indent or groove 228 will slide more freely in the chamber and reduces the risk of the piston jamming in the barrel. 25 Additionally, the double sealing effect of the double O- ring increases axial stability. The reduced diameter portion 218 of piston 210 has radial grooves 230 to aid in injection forming this part with one injection pin for removing the part from the mold. 30 The front collar 214 of piston 210 has a canted forward edge surface 732 for mating with a correspondingly canted shoulder 234 in the barrel when the piston is pushed forward in the chamber to mate with needle holder 202. As in the previous embodiments, the piston head 210 and needle 35 holder 202 have interengageable formations or grasping means which act to couple these parts together for retraction of the needle into the barrel after use. In this embodiment, the grasping means are different from the previous embodiments, and comprise a projection 236 at the forward end of the piston for engagement in a correspondingly shaped indent 238 at the rear end of the needle holder, rather than grasping fingers as in the previous embodiments. Projection 236 is of generally cylindrical shape and includes an annular ring portion 244 for engagement in a corresponding annular groove 253 in the indent. A flat or notch 248 is provided in ring portion 244. A conical forward end portion 246 projects forwardly from ring portion 244 for abutment with a corresponding tapered or frusto-conical surface portion 254 at the inner end of the indent. An annular groove 242 is located to the rear of ring portion 244, with a canted portion 240 extending between the front end of the piston and the groove 242. The indent 238 is of shape substantially matching that of the projection 236, and includes a tapered outer end portion 250, and annular rib portion 252 at its outer end for mating with groove 242. When the piston is urged forwardly so that the projection enters indent 238, rib portion 252 will be deformed slightly to allow the ring portion 244 to travel over the rib and snap into groove 253 as illustrated in Figure 35, coupling the two parts together. The remainder of needle holder 202 comprises a generally cylindrical portion 255 with a conically shaped hollow sheath portion 256 projecting from cylindrical portion 255 out of the forward end of the barrel which receives the needle housing 20, and an inner sheath or sleeve 258 projecting through sheath 256 and out of the forward end of sheath 256. The needle holder 202 has an axial through bore or passage 260 connecting the medication chamber 212 to the fluid passage of the needle and needle housing. Bore 260 communicates with the inner end of indent 238. The outer cylindrical surface of needle holder 202 includes an annular groove 262 adjacent the rear end of holder 202 in which an O-ring seal 264 is mounted. One or more flats 266 are provided in the outer diameter of holder 202 for assembly purposes and also for reducing the amount of material required to manufacture the holder. Outwardly projecting tabs 268 are formed at spaced intervals around the periphery of holder 202 at the forward end of cylindrical portion 256 for mating with corresponding slots 270 formed around the internal periphery of barrel 12 adjacent its forward end, as best illustrated in Figure 34. The interengaging tabs and slots act to restrict rotation of the needle holder in the barrel. The forward end 15 of the barrel has an in-turned lip 271 which acts as a stop against the forward end of cylindrical portion 255 to prevent the needle holder from being pulled forwardly out of the barrel. An annular groove or channel 273 extends from cylindrical portion 255 to outer sheath 256, and has an outer canted surface 275 to allow retraction of the needle holder when released. An annular groove 272 is formed in the conical sheath portion 256 for receiving the locking C-clip 204 in the locked position illustrated in Figures 33, 36 and 37. As in the version illustrated in Figure 24, the half-ring or C-clip 204 has a portion shaped to fit or snap-lock into groove 272. The circumference of ring 204 is slightly greater than one-half the circumference of groove 272. The forward end 15 of the barrel has a bore 274 projecting inwardly from its front end face parallel to the barrel axis for rotatably holding hinge pin 206 to which one end 276 of the C-clip is secured. As best illustrated in Figure 38, the hinge pin is of stepped diameter with stepped portions 277 of different sizes and shapes for axial retention of the pin in the bore. The opposite end 278 of the C-clip has a projecting finger tab 280 for use by an operator in urging the C-clip out of groove 272 to the released position illustrated in dotted outline in Figure 37. This insures that the C-clip is secured to the barrel even when released, reducing the amount of separate small pieces of debris produced in using syringes. Operation of syringe 200 will now be described in more detail. Prior to using the syringe, the C-clip 204 will be locked into groove 272 to prevent any movement of needle holder relative to barrel 12. The user normally first expels air from the syringe by pushing the piston forward in the barrel. The needle is then inserted into a supply of medication, and the piston is retracted backwardly in the chamber in order to draw medication through the needle 22, passage 260 and into the medication chamber 212. A small portion of drawn fluid is then ejected in order to remove any air from the chamber and to adjust to a metered volume of fluid in chamber 212. Fluid is then injected in the normal manner by pushing the piston forward, ejecting fluid from the chamber and out of needle 22. After the syringe has been used, the piston is urged further forward in the barrel so that projection 236 enters indent 238 to couple the piston and the needle holder together, simultaneously ejecting any remaining liquid from the chamber 212 into passage 260. The notch 248 on the ring portion of projection 236 ensures that fluid will not be trapped between the piston and needle holder as the ring 244 mates with groove 253, but can travel from the space 282 between the opposing front end face of the piston and rear end face of the needle holder and out to passage 260 via notch 248. The conical end portion 246 of projection 236 projects partially into the end of passage 260 in the engaged position illustrated in Figure 35, providing more stability to the needle holder as the assembly is retracted, and also reducing the amount of fluid trapped in the passage 260. The conical surface tends to direct fluid out via the central passage 260 as the projection enters the indent 238. Once the needle holder is secured to the piston, the user simply flips the C-clip out of channel or groove 272 into the released position illustrated in dotted outline in Figure 37. At this point the piston and needle holder can be retracted together into the barrel until the piston meets stop surface 226 at the rear end of the barrel, as illustrated in Figure 36. The needle holder and attached needle are therefore retracted cleanly into the barrel as in the previous embodiments, with the projecting end of the conical portion 246 acting to center the needle in the barrel as it is retracted. As in the previous embodiments, the projecting portion of the plunger shaft is snapped off at frangible portions 37, 38. The snapped off part of the shaft is then free and can be inserted in the open forward end of the barrel as illustrated in Figure 12, completely enclosing the needle and substantially reducing the risk of needle prick injuries and potential contamination or infection. The C-clip will prevent the needle holder from being accidentally retracted with the plunger during an injection procedure. Should the plunger accidentally be pushed forward with too much force when ejecting air from the barrel, so that the projection is urged into indent 238, retraction of the needle holder with the plunger will be prevented by the locking clip 204. The needle holder therefore cannot be retracted inadvertently into the barrel. If the plunger is coupled to the needle holder accidentally, the user simply exerts a larger pulling force on end member 16, pulling the piston sufficiently that the ring portion 244 is forced over the rib portion 252 of the indent, separating the piston from the needle holder. The syringe can then be operated normally as described above. The needle holder is made of a plastic which is tough yet has a sufficient degree of resilience to enable rib portion 252' to be deformed to allow the ring portion 244 to be forced over the rib portion 252 both when coupling the members together and if they should need to be separated during use of the syringe. The double O-ring seal on the piston head 210 has the additional advantage that the spaced surfaces of the seal wipe across the inner surface of the cylinder and act as wipers to squeeze any remaining liquid out of the barrel chamber 212. They also improve axial stability of both the piston and the needle holder when coupled to the piston, improving lateral stability of the needle as it is retracted. Figures 39-41 illustrate a modified version of the locking C-clip for locking the needle holder in the forward portion of the barrel during the normal injection procedure. In this version, other parts of the syringe are equivalent to those in the previous embodiment, and like reference numerals have been used where appropriate. However, instead of the C-clip having a hinge pin at one end for engagement in a bore projecting into the forward end of the barrel 12, the modified C-clip 304 of this embodiment has an indent or recess 306 at one end 308 which is of part-cylindrical shape and is designed to be snapped over a hinge post 310 projecting forwardly from the forward end of barrel 12, by pushing the recess over the post in a direction perpendicular to the post axis. As in the previous embodiment, C-clip 304 is of half-ring shape and has a portion shaped to snap-lock into groove 272 in the projecting portion 256 of the needle holder 202. The free end of C-clip 304 has a projecting finger tab 312 for use by an operator in urging C-clip 304 out of the groove to the released position illustrated in dotted outline in Figure 40. The hinge connection of recess 306 over hinge post 310 is illustrated in more detail in Figure 41. The hinge post 310 has a reverse taper 314, while the recess 306 in the C- clip has a matching taper 316. The mating tapered surfaces lock the C-clip against axial displacement from the post, while allowing the clip to rotated about the axis of post 310. This arrangement has been found to provide greater resistance to accidental separation of the C-clip from barrel 12, so that the C-clip is more likely to remain attached to barrel 12 after release, and not fall to the floor where it would cause unnecessary debris. Figures 42-52 illustrate another modified embodiment of the invention. In this embodiment, some parts are equivalent to parts in the previously illustrated embodiments, and like reference numerals have been used where appropriate. As illustrated in Figs. 42-52, a retractable syringe 400 of this embodiment comprises a hollow syringe barrel 320 having forward and rearward ends 322 and 324, respectively. A needle holder 402 is releasably locked in forward end 322 and may use a C-clip (not shown) , as described above with respect to other embodiments. Needle holder 402 is internally threaded at its forward end for receiving a standard end 64 of a needle housing 20 from which needle 22 projects. A plunger 408 includes a shaft 409 projecting into the rearward end 324 of the housing and a piston 410 at its forward end. Piston 410 is slidably disposed in barrel 320 for reciprocal movement in medication chamber 212 inside barrel 320. Plunger 408 has an enlarged thumb-driven actuator surface 326 at its rear end which coacts with finger grip 328 at rearward end 324 of barrel 320 to allow a user to reciprocate plunger 408 in barrel 320 in the desired fashion, in order to draw medication (not shown) into the medication chamber 330 of barrel 320 via needle 22 and eject medication back out through needle 22. Plunger 408 will now be described in more detail with reference to Figs. 43-48. Piston 410 is secured to the remainder of shaft 409 via a pair of frangible members 411 and 413. Frangible members 411 and 413 are similar to those described with respect to the preceding embodiments, but each comprises a reinforced portion 415 and a non- reinforced portion 417 having score lines 439. Reinforced portion 415 facilitates breakage along score lines 439 by reducing flexing of frangible members 411 and 413. Shaft 409 comprises a portion 419 having a generally U-shaped cross-section and portion 421 having a lath-like cross-section. U-shaped portion 419 comprises two sidewalls 423 and 425 that have stabilizing members 427 and 429, respectively. A pair of ring members or collars 431 and 433 are located between U-shaped portion 419 and lath¬ like portion 421 of shaft 409. Collars 431 and 433 are of slightly smaller dimensions than piston 410 and, as described above with respect to other embodiments, operate to center shaft 409 as piston 410 reciprocates in chamber 330. As shown in Figs. 51 and 52, needle 22 cannot contact any portion of shaft 409 when its end is inserted into forward end 322 of barrel 320 after needle holder 402 has been retracted and shaft 409 broken away from piston 410 because needle 22 is substantially centered within sidewalls 423 and 425 of U-shaped portion 419. Piston 410 is similar to piston 210 of the embodiment shown in Figs. 33-38 but, as described below, has a different structure for locking piston 410 to needle holder 402. Piston 410 includes a pair of spaced enlarged front and rear discs or collars 414 and 416 with a reduced diameter portion 418 extending between collars 414 and 416 for receiving split O-ring seal 422, which is similar to the O-ring- seal of Figs. 25 and 33 above. A notch 428 divides split O-ring seal 422 into halves that are preferably symmetric. Split O-ring seal 422 provides a double sealing effect that reduces leakage that may occur due to lateral deviations of shaft 409 from a centered orientation on a longitudinal axis along which it is reciprocated in chamber 330. Split O-ring seal 422 also has a stabilizing effect that minimizes lateral movement or wobble of needle 22 when needleholder 402 is in the retracted position after shaft 409 has been broken away. When shaft 409 is inserted into forward end 322 of barrel 320, this minimization of lateral movement maintains needle 22 centered within sidewalls 423 and 425 of U-shaped portion 419. Needle 22 cannot contact shaft 409 and thereby inhibit further insertion of shaft 409 because needle 22 remains centered, as shown in Figs. 51-52. As in the embodiment shown in Figs. 33-38, reduced diameter portion 418 of piston 410 has radial groove 430 to aid in injection forming this part with one ejection pin for removing the part from the mold. As in the embodiment described above with respect to Figs. 33-38, rearward end 324 of barrel 320 includes a reduced diameter portion 332 with a ramped or inclined shoulder 334 facing chamber 330 that acts as a stop against rear collar 416 to prevent piston 410 from being withdrawn from barrel 320, as illustrated in Fig. 51. However, forward end 322 of barrel 320 is different from forward end 15 of the embodiments discussed above. As shown in Figs. 49, 50 and 59, forward end 322 includes a plurality of inwardly facing protuberances 336 spaced around its circumference. The spaces between adjacent protuberances 336 define a plurality of slots 338. Needle holder 402 may have radially projecting tabs 468 similar to tabs 268 described above with respect to the embodiment shown in Figs. 33-38. The interengaging tabs 468 and slots 338 act to restrict rotation of needle holder 402 in barrel 320. Protuberances 336 may have pointed or tapered rearward ends 340 for guiding tabs 468 into slots 338 during assembly of syringe 400. As shown in Fig. 50, protuberances 336 are evenly spaced around the circumference. This even spacing combined with tapering of protuberances 336 avoids the step of rotationally aligning needle holder 402 with any particular slots 338 during assembly of syringe 400. As shown in Fig. 42, pointed rearward ends 340 may also act as stops to prevent needle holder 402 from being pulled forwardly out of barrel 320. Alternatively, or in addition, protuberances 336 may have inwardly turned lips 342 that act as stops in a similar manner. Forward end 322 has two bores 474 projecting rearwardly parallel to the barrel axis for rotatably holding a C-clip hinge pin (not shown) in the manner described above with respect to other embodiments. Unlike the above-described embodiments that have only a single such bore, the dual bores facilitate automated assembly of syringe 400 because bores 474 are symmetrically disposed with respect to finger portion 328. Thus, finger portion 328 alone can be used to orient syringe 400 in a machine that inserts a C-clip hinge pin into whichever one of bores 474 is properly aligned in the machine. As best shown in Figs. 43 and 48, front collar 414 of piston 410 has a canted forward edge surface 432 for mating with a correspondingly canted shoulder 434 in barrel 320 when piston 410 is pushed forward in chamber 330 to mate with needle holder 402. In an arrangement similar to that described above with respect to the embodiment shown in Figs. 33-38, the mating means comprises a projection 436 at the forward end of piston 410 for engagement in a correspondingly shaped indent 438 at the rear end of needle holder 402. Projection 436, shown in Figs. 43, 44, 57, and 58, is of generally cylindrical shape and, unlike other embodiments, includes a generally annular lip 444 for engagement in a corresponding annular groove 453 in indent 438 of needle holder 402. Lip 444 should have a substantially uniform thickness, but may have a forward surface 445 that is slightly canted. During the manufacturing of the steel mold (not shown) for lip 444 it is preferred to provide a semi-rough surface produced during the electrical discharge machining (EDM) process. This semi-rough surface acts to slow down the plastic melt flow, thereby stabilizing the filling of the cavity during the injection molding process and thus producing a uniform annular lip 444 that is molecularly sound. If the surface of the steel mold for lip 444 were polished to a smooth finish the plastic melt during the injection molding process would accelerate, thereby creating a swirl effect. This effect would produce a lip 444 with an irregular shape and a non-uniform molecular structure that is undesirable for the present invention. A conical forward end portion 446 projects forwardly from lip 444 for abutment with a corresponding tapered or frusto-conical surface portion 454 at the inner end of indent 438. A forward canted portion 443 extends between lip 444 and forward end portion 446. An annular notch or fillet 442, which defines an acute angle, is located between the rearward surface of lip 444 and a rearward canted portion 440. A canted stop portion 441 extends rearwardly of rearward canted portion 440. Two notches 448 are provided in projection 436 and extend through both canted portion 440 and lip 444, thereby dividing lip 444 in half. The notching of lip 448 facilitates its flexure, as described below. As shown in Figs. 44 and 58, when projection 436 is inserted into indent 438 to mate plunger 408 with needle holder 402, a reduced diameter neck 450 resiliently deforms lip 444, which deflects rearwardly as it passes through neck 450. Lip 444 may contact stop portion 441 during rearward deflection of lip 444, thereby preventing further deflection. After lip 444 passes through neck 450, lip 444 resiliently snaps into groove 453, which is between neck 450 and a stop ring 452. When engaged in groove 453, lip 444 assumes an arcuate rim shape and grips groove 453 from a rearward angle in a barb-like manner, as shown in Fig. 44. To assume this shape, lip 444 resiliently deforms in an angular direction. Thus, unlike the snap engagement of the embodiment described above with respect to Figs. 33-38, which deforms essentially in only a radial direction, the deformation of the outer perimeter of lip 444 has both a radial component and a longitudinal component. In its undeformed state, lip 444 is substantially perpendicular to the longitudinal axis of syringe 400, but lip 444 can preferably be resiliently deflected in a rearward direction by approximately 70 degrees when lip 444 passes through neck 450 and preferably springs back into groove 453 by approximately 25 percent of that amount. The preferred structure of lip 444 considers the tradeoff between achieving this deflection and maximizing the transfer of force from lip 444 to needle holder 402 during retraction. One factor that affects the tradeoff between deflection and maximization of force transfer is the radius size of fillet 442. Increasing the radius of fillet 442 increases the strength of lip 444. The angle formed between the forward surface of lip 444 and forward canted portion 443 affects this tradeoff in a similar manner. Another factor is the ratio of the diameter of lip 444 to that of groove 453. In its undeformed state, lip 444 preferably has a diameter that is between approximately four and seven percent larger than that of indent 438. Still another factor is the thickness of lip 444, which is preferably between approximately six and eight percent of the diameter of neck 450. Stop ring 452 abuts the forward surface of lip 444 when lip 444 is engaged in groove 453 and inhibits further deformation of lip 444 when a rearward force is applied to plunger 408 for retracting needle holder 402. This arrangement maximizes the force that can be transferred from plunger 408 to needle holder 402 in a rearward direction when plunger 408 is retracted without causing them to disengage because the frusto-conical rim shape of lip 444, reinforced by stop ring 452, provides substantial strength and rigidity. The remainder of needle holder 402 may comprise a structure similar to that described above with respect to Figs. 33-38. A generally cylindrical portion 455 with a conically shaped hollow sheath portion 456 projects from cylindrical portion 455 out of the forward end 322 of barrel 320, which receives needle housing 20. An inner sheath or sleeve 458 projects through sheath 456 and out of the forward end of sheath 456. The needle holder 402 has an axial through-bore or passage 460 connecting the medication chamber 330 of barrel 320 to the fluid passage of needle housing 20 and needle 22. Bore 460 communicates with the forward end of indent 438. The outer cylindrical surface of needle holder 402 includes an annular groove 462 adjacent the rear end of holder 402 in which an O-ring seal 464 is mounted. Syringe 400 may operate in the manner described above with respect to syringe 200 of another embodiment. As discussed above, syringe 400 may include a locking C-clip (not shown) for retaining and releasing needle holder 402. An annular groove 472 is formed in conical sheath portion 456 for receiving the C-clip, which is used to lock and unlock needle holder 402 and barrel 320, as described above with respect to other embodiments. After performing an injection, the user engages plunger 408 and needle holder 402 in the manner described above with respect to the embodiment shown in Figs. 33-38. The user then flips the C-clip out of groove 472 into the released position, and retracts plunger 408 and needle holder 402 together into barrel 320 until piston 410 meets inclined shoulder 334 at the rear end of the barrel. As in the above-described embodiments, shaft 409 is snapped free at frangible portions 439 from piston 410. The forward (U- shaped) end of shaft 409 can then be inserted in forward end 322 of barrel 320, as illustrated in Figs. 51-52. The edges of sidewalls 423 and 425 of U-shaped portion 419 slide into two of grooves 338 in forward end 322. Similarly, the edges of stabilizing members 427 and 429 of U-shaped portion 419 slide into two other grooves 338. Grooves 338 may have chamfers 473 at forward end 322 to facilitate insertion of U-shaped portion 419. The engagement of the edges of sidewalls 423 and 425 and stabilizing members 427 and 429 in grooves 338 prevents both rotational movement and lateral movement (wobble) of U-shaped portion 419 with respect to barrel 320. This stabilization of U-shaped portion 419 maintains it centered within barrel 320, thereby preventing contact with needle 22 as U-shaped portion 419 is inserted. Furthermore, utilizing the edges of sidewalls 423 and 425 allows the width of U-shaped portion 419 to be maximized, further reducing the likelihood of contacting needle 22. Inserted in barrel 320, U-shaped portion 419 of shaft 409 may enclose a portion of needle holder 402, such as needle 22 and needle housing 20. Shaft 409 is retained in barrel 320 by snapping collars 431 and 433 over lips 342 and forward end 322, as shown in Fig. 51. Alternatively, collars 431 and 433 may be spaced further apart to snap over the entire length of protuberances 336. Figures 53-56 illustrate modified embodiments of the invention that may be particularly suitable for smaller syringes, such as those designed to hold between 1 cubic centimeter (cc) and 3 cc of fluid. A syringe barrel 502 has a plurality of grooves 504 in its forward end. A needle holder 506 has a plurality of ribs 508 for mating with grooves 504. A C-clip 518 for retaining needle holder 506 in the forward end of barrel 502, as described above with respect to other embodiments, is seated in a bore 520. C-clip 518 engages a circumferential groove 522 in needle holder 506, as described above with respect to other embodiments. C-clip 518 has a tab 515 that engages one of grooves 504 to provide additional retention strength and security. Some of ribs 508 extend completely through the forward end of barrel 502. These ribs 508 are intersected or divided by groove 522. However, other ribs 508 extend forward from slot 522 but do not extend into barrel 502. To form slot 522 using an injection mold (not shown) , the halves of the mold may separate away from needle holder 506 in opposite radial directions. The number of ribs 508 that do not extend completely through the forward end of barrel 502 may exceed the number of the longer type of ribs 508, i.e., those ribs 508 that extend completely through the forward end of the barrel, because a large number of the longer ribs 508 may inhibit removal of needle holder 506 from the mold. Although needle holder 506 preferably has five or more ribs 508 that extend only forwardly from slot 522, needle holder 506 may have as few as one or two of the longer ribs 508. The longer ribs 508 are needed only for preventing rotational movement of needle holder 506. Grooves 504 may have chamfers 523 at the forward end of barrel 502 and ribs 508 may have pointed or tapered ends for guiding ribs 508 into grooves 504 during assembly of the syringe. In addition, chamfers 523 facilitate retraction of needle holder 506 by guiding the notch surfaces 524 of the longer ribs 508 through the forward end of barrel 502. As in the above-described embodiments, a piston 510 and a shaft 512 are disposed in barrel 502, and needle holder 506 includes needle housing 20 and needle 22. The grooved portion of barrel 502 has no structure similar to lips 342 described above with respect to a different embodiment. Rather, when shaft 512 is inserted into the forward end of barrel 502, the two collars 514 and 516 on shaft 512 snap over the entire grooved portion, as shown in Fig. 55. In this embodiment, the interengaging grooves 504 and ribs 508 not only prevent relative rotation of barrel 502 and needle holder 506, but also provide substantial strength and rigidity to needle holder 506. In small syringes, needle housing 20 may be almost as wide as the inner diameter of barrel 502, leaving little space between them to accommodate retraction of the internally threaded portion of the needle holder that receives needle housing 20 into the relatively small internal diameter of the barrel chamber 511. Therefore, in a small syringe of the type described above with respect to other embodiments, the internally threaded portion of the needle holder would have relatively thin and correspondingly weak walls. In this embodiment, the combination of ribs 508 and the slotted portion of barrel 502 provide substantial strength and rigidity despite the use of a needle holder having a relatively thin cross-section in the areas between ribs 508. As an alternative to mounting needle housing 20 in an internally threaded portion of needle holder 506, needle 22 may be potted or insert-molded in a needle holder 560, as illustrated in Fig. 56. This arrangement saves space within needle holder 560 and allows needle holders 560 having extremely small diameters to be manufactured. However, potting or insert-molding a needle into a needle holder is suitable for any size syringe. Needle holder 560 may have ribs 561 that engage corresponding slots similar to those described above with respect to the embodiment shown in Figs. 53-54. As in the above-described embodiments, a needle cover 562 may be removably attached to needle housing 20 using a suitable connection means, such as a friction fit. In any embodiment, syringes of the present invention may be assembled by inserting the needle holder into the rearward end of the barrel and urging it into engagement with the forward end. In the above-described embodiments, the needle and needle cover may then be attached to the portion of the needle holder that protrudes from the forward end of the barrel. However, in this embodiment needle 22 is permanently attached to needle holder 560 by potting or insert-molding it. It would be undesirable to insert a needle holder 560 having an exposed needle 22 into the barrel 502 during asssembly. Therefore, needle cover 562 may be attached to needle housing 20 before insertion, and the combined assembly consisting of needle holder 560, needle 22 and needle cover 562 may then be inserted into the rearward end (not shown) of barrel 502. Cover 562 has radial fins 564 around its base that provide strength and rigidity. Fins 564 should thus be aligned with ribs 561 to facilitate passage of fins 564 through the slots in the forward end of barrel 502. Although some preferred embodiments of the present invention have been described above by way of example only. it will be understood by those skilled in the field that modifications may be made to the disclosed embodiments without departing from the scope of the invention, which is defined by the appended claims. WE CLAIM:

Claims: CLAIMS 1. A retractable syringe comprising: a syringe barrel having an internal chamber with open forward and rearward ends and a piston and shaft in the barrel with one end of the shaft extending out the rearward open end; a needle holder and needle releasably held in the forward end of the barrel in a first position in which the needle holder projects partially out of the open forward end of the barrel; said piston being longitudinally movable in said barrel for retracting the needle holder from said first position to a retracted position in which the barrel encloses the needle holder and needle; said piston and needle holder having interengageable means for selectively securing said piston to said needle holder, said interengageable means being provided on a forward abutting end of said piston and a rearward abutting end of said needle holder, one of said abutting ends having a central recess and the other end having a projection for releasable snap engagement in said recess; and said recess having an internal circumferential groove and said projection having a flexible substantially annular lip for engaging said groove, said lip having an outer perimeter having a diameter, said groove having a diameter less than said diameter of said outer perimeter of said lip, and said outer perimeter of said lip being deflected in an angular direction when said lip is engaged in said groove. 2. A retractable syringe as claimed in Claim 1, wherein said lip is deflected at an angle of between 45 and 60 degrees when said lip is engaged in said groove. 3. A retractable syringe as claimed in Claim 1, wherein said shaft has a weakened section along its length for breaking said shaft. 4. A retractable syringe as claimed in Claim l, wherein: said barrel has adjacent said rearward opening an inward projecting member; and said piston has an outer surface for abutting against said inward member, blocking movement of said piston out the open end of said barrel. 5. A retractable syringe as claimed in Claim 1, wherein: said piston has an outer circular slot with an O-ring positioned in said slot; and said O-ring fitting against the inner surface of said barrel forming a movable seal. 6. A retractable syringe as claimed in Claim 5, wherein said O-ring positioned in the outer radial slot of said piston has a split outer radial surface with a radially inward space in said outer surface, providing a pair of separate outer resilient circumferential contacts between the O-ring and the inner surface of said barrel. 7. A retractable syringe as claimed in Claim 6, wherein said barrel has metering marks along the length thereof that coact with one of the edge surfaces of said outer radial space of said O-ring to provide metering indication of the fluid in said barrel. 8. A retractable syringe as claimed in Claim 1, wherein said needle holder and forward end of said barrel have interacting key and slot means for locking the needle holder against rotating movement in the forward end of the barrel. 9. The syringe as claimed in Claim 8, wherein said key and slot means comprise a plurality of circumferentially- spaced, radially-projecting tabs on said needle holder and plurality of corresponding slots on said barrel for receiving said tabs. 10. A retractable syringe as claimed in Claim 1, including: a sheath having a closed forward end and rear open end that fits over the forward end of the needle holder and needle; and the rear open end having a shoulder that abuts the forward end of said barrel, whereby when said sheath is positioned over said needle and needle holder, forces applied to said sheath are exerted against the end of the barrel. 11. A retractable syringe as claimed in Claim 1, wherein: said needle holder has a forward groove; said retractable syringe further comprises an arcuate lock clip for fitting into said groove and abutting the front end surface of said barrel; and whereby said lock clip positioned in said groove, prevents rearward movement of said needle holder. 12. A retractable syringe as claimed in Claim 11, wherein: said lock clip having a "C" shape with a slightly larger circumference than half the circumference of said groove for being laterally snapped in said slot and being resiliently retained in position; and one end of said clip has a projection that may be actuated to remove said lock clip from said slot. 13. The syringe as claimed in Claim 12, wherein said lock clip is hinged at one end to the forward end of said barrel for pivotal movement about said hinged end between a locking position in said groove and a released position out of said groove for permitting rearward movement of said needle holder. 14. The syringe as claimed in Claim 1, wherein: said projection has an at least partially conical portion extending therefrom, and said recess has a corresponding at least partially conical surface; and said at least partially conical portion of said projection extends into said recess when said lip is engaged in said groove. 15. The syringe as claimed in Claim 14, wherein said needle holder has an axial through bore communicating with said needle at the forward end of said needle holder and with said recess at the rear end of said needle holder. 16. The syringe as claimed in Claim 1, wherein said projection has at least one notch. 17. The syringe as claimed in Claim 16, wherein each said notch extends through said annular lip of said projection. 18. The syringe as claimed in Claim 1, wherein said shaft is breakable along its length with the broken end of the shaft inserted into the forward end of the barrel holding the needle holder and needle in the retracted position. 19. The syringe as claimed in Claim 18, wherein a portion of said shaft has a U-shaped cross-section and covers at least a portion of said needle when said shaft is inserted into the forward end of the barrel holding the needle holder and needle in the retracted position. 20. A retractable syringe as claimed in Claim 18, wherein said weakened section is adjacent to said piston. 21. A retractable syringe as claimed in Claim 20, wherein: said barrel has adjacent said rearward opening an inward projecting member; and said piston has an outer surface for abutting against said inward member, blocking movement of said piston out the open end of said barrel. 22. A retractable syringe as claimed in Claim 21, wherein: said piston has an outer circular slot with an O-ring positioned in said slot; and said O-ring fitting against the inner surface of said barrel forming a movable seal. 23. A retractable syringe as claimed in Claim 22, wherein said O-ring positioned in the outer radial slot of said piston has a split outer radial surface with a radially inward space in said outer surface, providing a pair of separate outer resilient circumferential contacts between the O-ring and the inner surface of said barrel. 24. A retractable syringe as claimed in Claim 23, wherein said. barrel has metering marks along the length thereof that coact with one of the edge surfaces of said outer radial space of said O-ring to provide metering indication of the fluid in said barrel. 25. A retractable syringe as claimed in Claim 18, wherein said needle holder and forward end of said barrel have interacting key and slot means for locking the needle holder against rotating movement in the forward end of the barrel. 26. The syringe as claimed in Claim 25, wherein said key and slot means comprise a plurality of circumferentially- spaced, radially-projecting tabs on said barrel, the spacing between said tabs on said barrel defining a plurality of grooves, and a plurality of radially- projecting tabs on said needle holder, and said tabs on said needle holder are received in said grooves on said barrel. 27. The syringe as claimed in Claim 26, wherein said grooves on said barrel are evenly spaced around the circumference of said barrel. 28. The syringe as claimed in Claim 27, wherein said tabs on said barrel have rearward ends tapering to a rearwardmost point for guiding said tabs on said needle holder into said grooves on said barrel. 29. A retractable syringe as claimed in Claim 18, including: a sheath having a closed forward end and rear open end that fits over the forward end of the needle holder and needle; and the rear open end having a shoulder that abuts the forward end of said barrel, whereby when said sheath is positioned over said needle and needle holder, forces applied to said sheath are exerted against the end of the barrel. 30. A retractable syringe as claimed in Claim 18, wherein: said needle holder has a forward groove; said retractable syringe further comprises an arcuate lock clip for fitting into said groove and abutting the front end surface of said barrel; and whereby said lock clip positioned in said groove, prevents rearward movement of said needle holder. 31. A retractable syringe as claimed in Claim 30, wherein: said lock clip having a "C" shape with a slightly larger circumference than half the circumference of said groove for being laterally snapped in said slot and being resiliently retained in position; and one end of said clip has a projection that may be actuated to remove said lock clip from said slot. 32. The syringe as claimed in Claim 31, wherein said lock clip is hinged at one end to the forward end of said barrel for pivotal movement about said hinged end between a locking position in said groove and a released position out of said groove for permitting rearward movement of said needle holder. 33. The syringe as claimed in Claim 18, wherein: said projection has an at least partially conical portion extending therefrom, and said recess has a corresponding at least partially conical surface; and said at least partially conical portion of said projection extends into said recess when said lip is engaged in said groove. 34. A retractable syringe as claimed in Claim 18, wherein said forward abutting end of said piston has said projection and said rearward abutting end of said needle holder has said recess. 35. The syringe as claimed in Claim 34, wherein said needle holder has an axial bore communicating with said needle at the forward end of said needle holder and with said recess at the rearward end of said needle holder. 36. The syringe as claimed in Claim 18, wherein said projection has at least one notch. 37. The syringe as claimed in Claim 36, wherein each said notch extends through said annular lip of said projection. 38. A retractable syringe, comprising: a syringe barrel with open forward and rearward ends and a piston and shaft in the barrel with one end of the shaft extending out the rearward open end; a needle holder and needle releasably held in the forward end of the barrel in a resiliently interengageable connection that holds the needle holder from longitudinal movement toward the rearward end to a position where the barrel encloses the needle holder and needle, said interengageable connection comprising a circumferential groove and an annular lip that resiliently fits into said groove, resiliently holding the needle holder in the forward end of the barrel; said piston and needle holder having interengageable formations for selectively securing the needle holder to the piston for pulling the needle holder out of said resiliently interengageable connection to the enclosed position in the barrel; and said piston having an outer circumferential slot with a split O-ring positioned in said slot, said split O-ring having a pair of separate outer resilient circumferential contacts between said split O-ring and the inner surface of said barrel, and said split O-ring fitting against the inner surface of said barrel forming a movable seal. 39. A retractable syringe as claimed in Claim 38, wherein: said interengageable formations are provided on a forward abutting end of said piston and a rearward abutting end of said needle holder, and one of said abutting ends has a central recess and the other end has a projection for releasable snap engagement in said recess; and said recess has an internal circumferential groove and said projection has a flexible substantially annular lip for engaging said groove, said lip has an outer perimeter having a diameter, said groove has a diameter less than said diameter of said outer perimeter of said lip, and said outer perimeter of said lip is deflected in at least a longitudinal direction when said lip is engaged in said groove. 40. A retractable syringe as claimed in Claim 39, wherein said forward abutting end of said piston has said projection and said rearward abutting end of said needle holder has said recess. 41. A retractable syringe as claimed in Claim 38, wherein said barrel has said annular lip and said needle holder has said circumferential groove. 42. A retractable syringe as claimed in Claim 38, wherein said slot has a canted outer surface for camming said needle holder over said annular lip on rearward movement of said needle holder. 43. A retractable syringe as claimed in Claim 38, wherein said outer circumferential slot of said piston contacts only substantially flat surfaces of said O-ring. 44. A retractable syringeas claimed in Claim 38, wherein said pair of separate outer resilient circumferential contacts are symmetric. 45. The syringe as claimed in Claim 38, wherein said shaft is breakable along its length with the broken end of the shaft inserted into the forward end of the barrel holding the needle -holder and needle in the retracted position. 46. The syringe as claimed in Claim 45, wherein said needle is integral with said needle holder. 47. A retractable syringe as claimed in Claim 45, wherein said needle holder and forward end of said barrel have interacting key and slot means for locking the needle holder against rotating movement in the forward end of the barrel. 48. The syringe as claimed in Claim 47, wherein said key and slot means comprise a plurality of circumferentially- spaced, radially-projecting tabs on said barrel, the spacing between said tabs defining a plurality of grooves, and a plurality of radially-projecting tabs on said needle holder, and said tabs on said needle holder mesh with said grooves on said barrel. 49. The syringe as claimed in Claim 48, wherein said grooves on said barrel are evenly spaced around the circumference of said barrel. 50. The syringe as claimed in Claim 49, wherein said tabs on said barrel have rearward ends tapering to a rearwardmost point for guiding said tabs on said needle holder into said grooves on said barrel. 51. The syringe as claimed in Claim 48, wherein a portion of said shaft has a U-shaped cross-section and covers at least a portion of said needle when said shaft is inserted into the forward end of the barrel holding the needle holder and needle in the retracted position. 52. The syringe as claimed in Claim 51, wherein said portion of said shaft having said U-shaped cross-section has longitudinally extending projections that engage said slots on said barrel when said shaft is inserted into the forward end of the barrel holding the needle in the retracted position. 53. A retractable syringe, comprising: a syringe barrel with open forward and rearward ends and a piston and shaft in the barrel with one end of the shaft extending out the rearward open end; a needle holder and needle releasably held in the forward end of the barrel in a first position in which the needle holder projects partially out of the open forward end of the barrel; said needle holder and forward end of said barrel have interacting key and slot means for locking the needle holder against rotating movement in the forward end of the barrel, said key and slot means comprising a plurality of circumferentially spaced, radially inwardly projecting tabs on said barrel, and a plurality of circumferentially spaced, radially outwardly projecting tabs on said needle holder, the spacing between said tabs on said barrel defining a plurality of grooves for meshing with said tabs on said needle holder; said piston and needle holder having interengageable means for selectively securing said piston to said needle holder; and said piston being longitudinally movable in said barrel for retracting said needle holder from said first position to a retracted position in which said barrel encloses said needle holder and needle. 54. The syringe as claimed in Claim 53, wherein said needle is integral with said needle holder. 55. The syringe as claimed in Claim 53, wherein said tabs on said needle holder extend from said forward end of said needle holder to said forward end of said barrel. 56. The syringe as claimed in Claim 53, wherein said grooves on said barrel are evenly spaced around the circumference of said barrel. 57. The syringe as claimed in Claim 56, said tabs on said barrel have rearward ends tapering to a rearwardmost point for guiding said tabs on said needle holder into said grooves on said barrel. 58. The syringe as claimed in Claim 56, wherein said plurality of tabs on said needle holder is at least five. 59. The syringe as claimed in Claim 58, wherein said needle is integral with said needle holder. 60. The syringe as claimed in Claim 53, wherein: 2 said interengageable means is provided on a forward abutting end of said piston and a rearward abutting end of 4 said needle holder, one of said abutting ends has a central recess and the other end has a projection for releasable 6 snap engagement in said recess; and said recess has an internal circumferential groove and 8 said projection has a flexible substantially annular lip for engaging said groove, said lip has an outer perimeter 10 having a diameter, said groove has a diameter less than said diameter of said outer perimeter of said lip, and said 12 outer perimeter of said lip is deflected in at least a longitudinal direction when said lip is engaged in said 14 groove. 61. The syringe as claimed in Claim 60, wherein said i grooves on said barrel are evenly spaced around the circumference of said barrel. 62. The syringe as claimed in Claim 61, said tabs on said > barrel have rearward ends tapering to a rearwardmost point for guiding said tabs on said needle holder into said L grooves on .said barrel. 63. The syringe as claimed in Claim 61, wherein said 1 plurality of tabs on said needle holder is at least five. 64. The syringe as claimed in Claim 63, wherein said 2 needle is integral with said needle holder.

Inventors: MANJARREZ CARLOS H, MAZUR MATTHEW S, MANJARREZ, CARLOS, H., MAZUR, MATTHEW, S.
Applicants: US MEDICAL INSTR INC, U.S. MEDICAL INSTRUMENTS, INC.
Application year: 1994
Main classification: A61M5
IPC codes: A61M5